Early Cell Cycle Box-Mediated Transcription of CLN3 and SWI4 Contributes to the Proper Timing of the G sub(1)-to-S Transition in Budding Yeast

The Cln3-Cdc28 kinase is required to activate the Swi4-Swi6 transcription complex which induces CLN1 and CLN2 transcription in late G sub(1) and drives the transition to S. Cln3 and Swi4 are both rate limiting for G sub(1) progression, and they are coordinately transcribed to peak at the M/G sub(1) boundary. Early cell cycle box (ECB) elements, which confer M/G sub(1)-specific transcription, have been found in both promoters, and elimination of all ECB elements from the CLN3 promoter causes both a loss of periodicity and Cln3-deficient phenotypes, which include an extended G sub(1) interval and increased cell volume. Mutants lacking the ECB elements in both the CLN3 and SWI4 promoters have low and deregulated levels of CLN transcripts, and the G sub(1)-to-S transition for these mutants is delayed and highly variable. These observations support the view that the coordinated rise of Cln3 and Swi4 levels mediated by ECB-dependent transcription controls the timing of the G sub(1)-to-S phase transition.